It is said that during mass migrations birds rely on three senses. Firstly, they hold a mental map of the terrain from a high vantage point, and thus can visually guide themselves on.
Secondly, a sixth sense seems to use the earth’s magnetic field, or rather the fluctuations in it, to keep a sense of location during flight.
Thirdly, birds have been proven to associate certain smells with their home region. Aromas drifting for miles high up at the level of bird flight can be detected by these creatures. The identification of the scent- it could be of the sea-coast, or of an industrial landscape, and a judgement of the prevailing wind direction, can help the bird home in on home.